1. Field of the Invention
The present invention relates to a purging agent to rapidly purge thermoplastic resins, especially those which contain ethylene-vinyl alcohol copolymer (EVOH), from melt passages after a molding operation. Only a little of this purging agent remains after use, and therefore causes virtually no defects after the reintroduction of a fresh thermoplastic resin into the passages.
2. Description of the Background
EVOH has gained wide acceptance as a packaging film and for containers, because of its good oxygen and flavor barrier properties, which are required for some types of foods and articles.
Unfortunately, EVOH is subject to gelation and decomposition. This occurs in the pocket of the extrusion passage, after long periods of extrusion operation. This may give rise to undesirable matter, which manifests itself as stones and streaks or as irregular thickness in the extrusion product, giving the product a poor appearance. Moreover, EVOH gels and decomposes with foaming in the passage when operation is resumed, with the result that yellowed EVOH or gelled EVOH emerges continuously, requiring long periods of time until acceptable products are obtained. In the worst cases, it is necessary to disassemble the equipment for cleaning. This same problem also arises in molding operations, after switching from EVOH to another resin (such as polyolefin, polyamide, polyester and polycarbonate) or another grade of EVOH (differing in ethylene content). The problem is particularly serious when switching to polyamide, polyester or polycarbonate, which are readily reactive with EVOH.
One way to address this problem is by purging the passages of EVOH with high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), polystyrene (PS) or poly-propylene (PP). Purging is carried out whenever any anomaly (stones and/or streaks) occurs during operation, or when operation is suspended. For effective purging, a resin (as the purging agent) having a low melt flow rate of about 0.05-10 g/10 min (at 190.degree. C. under a load of 2160 g) is commonly used. Another purging method is by staged switching to grades having a lower melt flow rate, by reduction of the extrusion temperature, or increase of the output.
The purging method mentioned above requires a complex procedure, takes a long time to carry out, or requires a large amount of purging agent. In addition, when EVOH molding is resumed after purging, the purging agent continues to enter the product, requiring long periods of time until acceptable products are obtained. Such residual purging agent manifests itself as stones and streaks which are a great hindrance to transparent multilayered objects produced from EVOH by coextrusion or coinjection molding with PS, polyethylene terephthalate (PET), polycarbonate (PC) or the like. If multilayered objects are drawn, such stones and streaks break the EVOH layer, resulting in pinholes.
Removal of gels of EVOH in the passage is achieved most effectively by using a purging agent having a low melt flow rate. However, such a purging agent is difficult to discharge once EVOH molding is resumed; it remains in the passage and continues to enter the products for a long time. One way measure to cope with this situation is to repeatedly switch from one purging agent to another having a higher melt flow rate. However, this is undesirable because it requires a complex procedure, takes a long time, and requires a large amount of purging agent.
Using a blend of polyamide and polyether elastomer as a purging agent is disclosed in Japanese Patent Laid-open No. 178545/1989. This method offers the advantage that the time required for acceptable products to be obtained is short after resumption of EVOH molding. This purging method is, however, unsatisfactory.
Japanese Patent Laid-open No. 269754/1993 discloses the use of an EVOH-based purging agent combined with an acid substance and a metal salt, in a specific ratio. This purging agent permits acceptable products to be obtained a short time after resumption of EVOH molding. However, it does not efficiently remove resin present in the pocket of the resin passage. Likewise, Japanese Patent Laid-open No. 279518/1993 discloses the use of a polyolefin-based purging agent combined with a metal salt. Its effectiveness is offset by the fact that repeated molding and purging for a long period of time results in the accumulation of gelled resin in the pocket of the resin passage, and requires a prolonged time before satisfactory production. (This will be demonstrated by a Comparative Example given later.)
Japanese Laid-Open Patent No. 308899/1990 discloses a detergent for molding machines which is composed of a thermoplastic resin having a specific melt flow rate and a prescribed amount of water. However, it describes neither EVOH nor problems with melt stability characteristic of EVOH.